Power circuit

ABSTRACT

A power circuit of an embodiment includes an amplifier circuit having a first and a second input. The amplifier circuit receives power from a power input and outputs an output voltage corresponding to a voltage difference between the first and second inputs. A reference voltage circuit supplies a reference voltage to the first input. A feedback circuit supplies a feedback voltage corresponding to the output voltage to the second input. A first ballast capacitance element is between the power input and the first input of the amplifier circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2016-175804, filed Sep. 8, 2016, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a power circuit.

BACKGROUND

In the related art, power circuits including switching regulators orlinear regulators are used to supply appropriate power-supply voltagesto a number of devices included in electronic apparatuses. When linearregulators receive variations in power-supply input to amplifiers(hereinafter referred to as noise), the linear regulators havecapabilities to reduce the noise. Power supply ripple rejection ratio(PSRR) characteristics are known as an index indicating a capability toremove noise in a linear regulator. The PSRR characteristics areexpressed as a ratio of a variation in input voltage to a variation inan output voltage from the regulator. The variation of the outputvoltage to a variation (noise) of the power-supply voltage or thereference voltage becomes smaller, and the resistance to noise isimproved, as the PSRR becomes higher. To improve the PSRRcharacteristics, methods of stabilizing a reference voltage source of alinear regulator, adjusting frequency characteristics of an open-loop,or increasing a gain have been considered. In these methods, however, aproblem occurs in that a circuit area of the linear regulator increasesor the current consumption increases.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating an example of a configurationof a linear regulator according to a first embodiment.

FIG. 2 is a cross-sectional view illustrating an example of aconfiguration of a MIS capacitor.

FIGS. 3A and 3B are diagrams illustrating examples of a configuration ofwiring capacitance.

FIGS. 4A to 4E are graphs illustrating waveforms of noise.

FIG. 5 is a graph illustrating PSRR characteristics of a linearregulator according to the first embodiment.

FIG. 6 is a circuit diagram illustrating an example of a configurationof a linear regulator according to a second embodiment.

FIGS. 7A to 7E are graphs illustrating waveforms of noise.

DETAILED DESCRIPTION

In general, according to one embodiment, a power circuit includes anamplifier circuit having a first input and a second input. The amplifiercircuit receives power from a power input and outputs an output voltagethat corresponds to a voltage difference between the first and secondinputs. A reference voltage circuit supplies a reference voltage to thefirst input of the amplifier circuit. A feedback circuit supplies afeedback voltage that corresponds to the output voltage to the secondinput of the amplifier circuit. A first ballast capacitance element isbetween the power input and the first input of the amplifier circuit.

In some examples, the first ballast capacitance element is providedhaving a capacitance value between the power input and the first inputsuch that a reciprocal (1/PSRR) of a power supply rejection ratio (PSRR)of the amplifier circuit is closer to 0 than when the first ballastcapacitance element is not provided.

Hereinafter, exemplary embodiments will be described with reference tothe drawings. The present disclosure is not limited to the exemplaryembodiments.

First Embodiment

In general, it is necessary to supply power (direct-current power) at anappropriate voltage for each device (for example, a microcomputer, asensor, and a driver) that is included in an electronic apparatus, suchas a mobile terminal. A power circuit, such as a switching regulatorand/or a linear regulator, is used to convert the power-supply voltageinto a desired voltage.

Power conversion efficiency of a switching regulator is generally good.Therefore, when a power-supply voltage is lowered, the power loss willbe small. However, a switching regulator tends to cause ripples (avoltage variation, hereinafter referred to as noise) in the power-supplyvoltage due to switching of a semiconductor switching element.

A linear regulator is typically inferior to a switching regulator inpower conversion efficiency since any voltage difference between aninput voltage and an output voltage is dissipated as heat. However, alinear regulator can often supply power with smaller noise since ripplesthat might be caused by switching do not occur.

Accordingly, a power circuit including a switching regulator and alinear regular is connected such that the switching regulator is usuallyconnected closer to the power supply than the linear regulator. In thiscase, a power-supply voltage can be efficiently first lowered by theswitching regulator and then subsequently converted into a power-supplyvoltage with small noise as is appropriate for each device (load) by thelinear regulator. Thus, the power circuit can supply the power havingsmall noise to the load with high power conversion efficiency.

When a voltage input to the power circuit and a voltage output from thepower circuit is small or a current output from the power circuit issmall, power may be converted only by the linear regulator withoutintervention of the switching regulator. In this case, this is becausepower conversion efficiency is not so important since supply power islow.

Such a linear regulator has a capability to reduce noise included inpower from the switching regulator or power from a power supply. Here,either power (or a voltage) from the switching regulator or power (or avoltage) from the power supply may be referred to as power-supply power(or power-supply voltage) from the linear regulator, and thus are alsocollectively referred to as power-supply power (or power-supplyvoltage). The capability to remove noise is expressed with a powersupply ripple rejection ratio (PSRR). The PSRR is expressed with a ratioof a variation value of a power-supply voltage to a variation value ofan output voltage. A variation in an output voltage to a variation(noise) of a power-supply voltage is smaller as the PSRR becomes higher.That is, it can be said that a linear regulator with a high PSRR hashigh resistance to power-supply noise.

FIG. 1 is a circuit diagram illustrating an example of the configurationof a linear regulator according to a first embodiment. A linearregulator 1 includes an amplifier circuit AMP, a reference voltagecircuit REF, a feedback circuit FB, and a first capacitance elementC_(BP).

The amplifier circuit AMP includes a first input IN1, a second inputIN2, a power input IN_(POW), and an output unit OUT. The amplifiercircuit AMP receives power-supply power from the power input IN_(POW)and outputs an output voltage (first voltage) V_(O) according to avoltage difference between the first input IN1 and the second input IN2to the output OUT. The first input IN1 inputs a reference voltage V_(P)from the reference voltage circuit REF and the second input IN2 inputs afeedback voltage V_(N) from the feedback circuit FB. Thus, the amplifiercircuit AMP has a function of adjusting the output voltage V_(O) so thatthe feedback voltage V_(N) is the same as the reference voltage V_(P)and of maintaining the adjusted output voltage V_(O). The amplifiercircuit AMP may be, for example, a differential amplifier circuit inwhich transistors provided on a semiconductor substrate are used. Here,Z_(AMP) is an output impedance of the amplifier circuit AMP.

The reference voltage circuit REF generates a voltage V_(DC) that doesnot depend on a power-supply voltage or temperature. The referencevoltage circuit REF is connected to the first input IN1 of the amplifiercircuit AMP and inputs the reference voltage V_(P) to the first inputIN1. The reference voltage circuit REF has output impedance Z_(DC), andinputs the generated voltage V_(DC) as reference voltage V_(P) to thefirst input IN1. The reference voltage circuit REF may be, for example,a band gap type power circuit.

The feedback circuit FB is connected between the output OUT and thesecond input IN2 and feeds a feedback voltage corresponding to theoutput voltage back to the second input IN2. The feedback circuit FBincludes a first resistance element R1 and a second resistance elementR2. The first resistance element R1 and the second resistance element R2are connected in series between the output OUT and a low voltage supplyGND. The first resistance element R1 is connected between the lowvoltage supply GND and a node ND, and the second resistance element R2is connected between the node ND and the output OUT. The node ND iselectrically connected to the second input IN2 and feeds the feedbackvoltage V_(N), as obtained by dividing the output voltage V_(O) by thefirst resistance element R1 and the second resistance element R2, backto the second input IN2. The first resistance element R1 and the secondresistance element R2 may be, for example, wiring resistors or diffusionlayer resistors. The low voltage supply GND may be lower than an inputvoltage V_(POW) to be input to the power input IN_(POW) or may be, forexample, a ground voltage supply (ground).

The first capacitance element C_(BP) is connected between the powerinput IN_(POW) and the first input IN1. The first capacitance elementC_(BP) is provided as ballast capacitance to prevent noise from beinggenerated in the output voltage V_(O) of the amplifier circuit AMP byparasitic capacitances C_(SN), C_(SP), and C_(SO) and to improve thepower supply ripple rejection ratio (PSRR) of the amplifier circuit AMP.The first capacitance element C_(BP) may be, for example, a metalinsulator semiconductor (MIS) capacitor or may be wiring capacitance.

FIG. 2 is a cross-sectional view illustrating an example of theconfiguration of a MIS capacitor. When a MIS capacitor is used as thefirst capacitance element C_(BP), the first capacitance element C_(BP)includes a substrate 10, a gate insulation film (first insulation film)20, and a gate electrode 30. The substrate 10 is, for example, asemiconductor substrate, such as a doped silicon substrate, and servesas a conductor. The gate insulation film 20 is provided on the substrate10 and may be formed of, for example, an insulating material such as asilicon oxide film. The gate electrode 30 is provided on the gateinsulation film 20 and may be formed of, for example, a conductivematerial such as polysilicon or metal. The substrate 10, serving as thefirst electrode, is electrically connected to the first input IN1. Thegate electrode 30 serving as the second electrode is electricallyconnected to the power input IN_(POW). Thus, the first capacitanceelement C_(BP) functions as capacitance between the power input IN_(POW)and the first input IN1. The capacitance of the first capacitanceelement C_(BP) may be adjusted by adjusting the thickness of the gateinsulation film 20 and/or a facing area of the gate electrode 30 and thesubstrate 10. The gate insulation film 20 may be formed of the samematerial as a MISFET gate insulation film provided on the substrate 10.The gate electrode 30 may be formed of the same material as a MISFETgate electrode provided on the substrate 10.

FIGS. 3A and 3B are diagrams illustrating examples of the configurationof a wiring capacitance. When wiring capacitance is used as the firstcapacitance element C_(BP), the first capacitance element C_(BP)includes a power wiring 40, a first wiring 50, and an insulation film(second insulation film) 60. The power wiring 40 extends from the powerinput IN_(POW) to the amplifier circuit AMP. For example, the powerwiring 40 may be a wiring between a power terminal of the linearregulator 1 receiving input power (power-supply power) from a powersupply or a switching regulator (not illustrated) and a power terminalof the amplifier circuit AMP. The first wiring 50 is a wiring betweenthe reference voltage circuit REF and the first input IN1. The powerwiring 40 and the first wiring 50 are both formed of, for example, aconductive material such as polysilicon or metal. The insulation film 60is provided between the power wiring 40 and the first wiring 50 andelectrically insulates the power wiring 40 from the first wiring 50. Theinsulation film 60 may be, for example, an insulating material such as asilicon oxide. The power wiring 40 and the first wiring 50 extend insubstantially parallel with each other for at least some portion oftheir length. Thus, the power wiring 40, the first wiring 50, and theinsulation film 60 form a capacitance between the power input IN_(POW)and the first input IN1. The capacitance of the first capacitanceelement C_(BP) may be adjusted by changing a distance between the powerwiring 40 and the first wiring 50 (i.e., the thickness of the insulationfilm 60) or the lengths of the portions of the power wiring 40 and thefirst wiring 50 that are substantially parallel to each other.

The power wiring 40 and the first wiring 50 may extend in substantiallyparallel in a straight line manner, as illustrated in FIG. 3A, or mayextend in substantially parallel in a staggered disposition(serpentine), as illustrated in FIG. 3B.

The power wiring 40, the first wiring 50, and the insulation film 60illustrated in FIGS. 3A and 3B may be arranged in substantially parallel(horizontal direction) to the surface of the substrate 10 on which theamplifier circuit AMP is provided or they may be stacked in thesubstantially perpendicular direction (vertical direction) to thesurface of the substrate 10. That is, the power wiring 40 and the firstwiring 50 may be wiring provided in the same layer or may be stackedwirings in different layers. The capacitance of the first capacitanceelement C_(BP) will be described below.

In the foregoing configuration, the linear regulator 1 according to thefirst embodiment functions to output the substantially constant outputvoltage V_(O) from the output unit OUT to a load (not illustrated).

Here, the parasitic capacitances C_(SN), C_(SP), and C_(SO) will bedescribed.

The parasitic capacitance C_(SN) occurs between the power input IN_(POW)and the second input IN2 and includes, for example, inter-electrodecapacitance or inter-wiring capacitance of transistors included in theamplifier circuit AMP. Noise from the input voltage V_(POW) is generatedin (is mixed with) the output voltage V_(O) via the parasiticcapacitance C_(SN) and the second resistance element R2 in some cases.

The parasitic capacitance C_(SP) occurs between the power input IN_(POW)and the first input IN1 and includes, for example, inter-electrodecapacitance or inter-wiring capacitance of transistors included in theamplifier circuit AMP. Noise of the input voltage V_(POW) is dividedbetween the output impedance Z_(DC) of the reference voltage circuit REFand the parasitic capacitance C_(SP) and is generated in (is mixed with)the reference voltage V_(P) is some cases. In this case, the noise ofthe input voltage V_(POW) is also generated in the output voltage V_(O).

The parasitic capacitance C_(SO) occurs between the power input IN_(POW)and the output OUT and includes, for example, inter-electrodecapacitance or inter-wiring capacitance of transistors included in theamplifier circuit AMP. Noise of the input voltage V_(POW) is dividedbetween the output impedance Z_(AMP) of the amplifier circuit AMP andthe parasitic capacitance C_(SO) and is generated in (is mixed with) theoutput voltage V_(O) in some cases.

In this way, the noise of the input voltage V_(POW) is generated in theoutput voltage V_(O) via the parasitic capacitances C_(SN), C_(SP), andC_(SO). The mixing of the noise is a cause of deterioration in noiseremoval characteristics (that is, the PSRR) of the linear regulator.

Power noise generated in the output voltage V_(O) is obtained bysuperposition of the noise originating from the parasitic capacitancesC_(SN), C_(SP), and C_(SO). A phase of the noise transmitted via theparasitic capacitances C_(SN), C_(SP), and C_(SO) is advanced by 90degrees from a phase of the noise of the input voltage V_(POW).

FIGS. 4A to 4E are graphs illustrating waveforms of noise. The verticalaxis represents the magnitude of a noise component (voltage) and thehorizontal axis represents a phase (time). FIG. 4A illustrates noise ofthe input voltage V_(POW). FIG. 4B illustrates noise transmitted via theparasitic capacitance C_(SP) in the output OUT. FIG. 4C illustratesnoise transmitted via the parasitic capacitance C_(SN) in the secondinput IN2 and the output OUT. In FIG. 4C, a dotted line indicates thenoise in the second input IN2 and a solid line indicates the noise inthe output OUT. FIG. 4D illustrates noise transmitted via the parasiticcapacitance C_(SO) in the output OUT. FIG. 4E illustrates noise (ballastnoise) transmitted via the first capacitance element C_(BP) in theoutput OUT. The waveforms of the noise in FIGS. 4A to 4E are depicted tofacilitate the understanding conveniently and may be different fromwaveforms of actual noise in real device.

A phase of the noise of the input voltage V_(POW) is advanced by 90degrees when the noise is transmitted via the capacitance (C_(SP),C_(SN), C_(SO), or C_(BP)). However, when noise is input to an invertedinput terminal as in the second input IN2 in FIG. 1, as illustrated inFIG. 4C, the phase of the noise is inverted at 180 degrees in the outputOUT. That is, the phase of the noise of the input voltage V_(POW) isadvanced by 90 degrees by the parasitic capacitance C_(SN), and thenfurther is inverted at 180 degrees from the inverted input (the secondinput IN2) of the amplifier circuit AMP and is transmitted to the outputOUT.

On the other hand, as illustrated in FIG. 4B, the phase of the noise inthe first input IN1 is not inverted in the output unit OUT. That is, thephase of the noise of the input voltage V_(POW) is transmitted to theoutput OUT in a state in which the phase of the noise is advanced by 90degrees by the parasitic capacitance C_(SP). As illustrated in FIG. 4D,the phase of the noise transmitted to the output voltage V_(O) via theparasitic capacitance C_(SO) is also transmitted to the output OUT in astate in which the phase of the noise is advanced by 90 degrees by theparasitic capacitance C_(SO).

In this way, the phase of the noise by the parasitic capacitance C_(SN)is inverted with respect to the phase of the noise by the parasiticcapacitances C_(SP) and C_(SO). Since the noise mixed in the outputvoltage V_(O) by the parasitic capacitances C_(SP), C_(SN), and C_(SO)is a sum of a curve of FIG. 4B, a curve of FIG. 4D, and a solid curvedline of FIG. 4C, the noise by the parasitic capacitance C_(SN) and thenoise by the parasitic capacitances C_(SP) and C_(SO) operate to becancel each other.

Normally, the parasitic capacitances C_(SP), C_(SN), and C_(SO) are notintentionally provided, but are accidentally occurring or unavoidablecapacitance. Accordingly, it would be merely an accident andconsiderably rare that the absolute value of the noise by the parasiticcapacitance C_(SN) would be the same as the absolute value of the noiseby the parasitic capacitances C_(SP) and C_(SO) and such that noise ofthe output voltage V_(O) would not be generated.

However, in this embodiment, the first capacitance element C_(SP) isintentionally provided as ballast capacitance to prevent noise beingmixed in the output voltage V_(O) due to the parasitic capacitancesC_(SP), C_(SN), and C_(SO). For example, when the absolute value of thenoise by the parasitic capacitance C_(SN) is greater than the absolutevalue of the noise by the parasitic capacitances C_(SP) and C_(SO), asillustrated in FIG. 1, the first capacitance element C_(BP) can beconnected between the power input IN_(POW) and the first input IN1 andprovided in parallel to the parasitic capacitance C_(SP). Thus, aballast noise component illustrated in FIG. 4E is also added to theoutput voltage V_(O), and thus the total sum of the noise in the outputvoltage V_(O) is reduced in magnitude.

For example, when NV_(OP) is a noise component mixed in the outputvoltage V_(O) by the parasitic capacitance C_(SP) and the firstcapacitance element C_(SP), NV_(OP) is expressed by Expression 1:

NV _(OP)=(1+R ₂ /R ₁)×NV _(P)  Expression 1:

where NV_(P) is assumed to be a noise component mixed in the first inputIN1 by the parasitic capacitance C_(SP) and the first capacitanceelement C_(BP). In the amplifier circuit AMP, the feedback voltage V_(N)is assumed to be the same as the reference voltage V_(P).

NV_(P) is expressed by Expression 2:

NV _(P) ={Z _(DC)/(Z _(DC)+1/(jω(C _(SP) +C _(BP))))}×V_(POW)  Expression 2:

Here, j is a complex number, ω is 2πf, and f is a frequency of noise.

Expression 3 is established based on Expressions 1 and 2:

NV _(OP)=(1+R ₂ /R ₁)×[{Z _(DC)/(Z _(DC)+1/(jω(C _(SP) +C _(BP))))}×V_(POW)]  Expression 3:

Here, a phase of NV_(OP) is advanced by 90 degrees from V_(POW) by theparasitic capacitance C_(SP) and the first capacitance element C_(BP).

When NV_(ON) is a noise component mixed in the output voltage V_(O) fromthe second input IN2 by the parasitic capacitance C_(SN), NV_(ON) isexpressed by Expression 4:

NV _(ON) =−R ₂/(1/(jωC _(SN)))×V _(POW)  Expression 4:

Here, a phase of NV_(ON) is delayed by 90 degrees as compared to V_(POW)(an inverted state with respect to the phase of NV_(OP)), as described.That is, when V_(POW) is a positive voltage, NV_(ON) is a negativevoltage component.

When NV_(OO) is a noise component mixed in the output voltage V_(O) fromthe amplifier circuit AMP by the parasitic capacitance C_(SO), NV_(OO)is expressed by Expression 5:

NV _(OO) ={Z _(AMP)/(Z _(AMP)+1/(jωC _(SO)))}×V _(POW)  Expression 5:

Here, a phase of NV_(OO) is advanced by 90 degrees from V_(POW). Thatis, when V_(POW) is a positive voltage, NV_(OO) is a positive voltagecomponent.

A noise component NV_(O) (NV_(OP)+NV_(ON)+NV_(OO)) generated in theoutput voltage V_(O) by the parasitic capacitances C_(SP), C_(SN), andC_(SO) and the first capacitance element C_(BP) is expressed byExpression 6:

NV _(O)={(1+R ₂ /R ₁)Z _(DC)/(Z _(DC)+1/(jω(C _(SP) +C _(BP))))−R₂/(1/(jωC _(SN)))+Z _(AMP)/(Z _(AMP)+(1/jωC _(SO)))}×V_(POW)  Expression 6:

In order to bring the noise component NV_(O) close to zero, the absolutevalue of the right side of Expression 6 should approach zero. Areciprocal (l/PSRR) of PSRR is expressed by Expression 7:

1/PSRR=∂NV _(O) /∂V _(POW)={(1+R ₂ /R ₁)Z _(DC)/(Z _(DC)+1/(jω(C _(SP)+C _(BE))))−R ₂/(1/(jωC _(SN)))+Z _(AMP)/(Z _(AMP)+(1/jωC_(SO)))}  Expression 7:

By bringing the absolute value of the right side of Expression close tozero, it is possible to obtain high PSRR characteristics.

Here, when the first capacitance element C_(BP) is not provided, amethod of bringing, for example, one or a plurality of C_(SP), C_(SN),C_(SO), Z_(DC), and Z_(AMP) close to zero can be used to bring theabsolute value of the right side of Expression 7 close to zero. However,in this method, there is a concern with increasing a circuit area or acurrent consumption, as described above.

Accordingly, in the embodiment, the first capacitance element C_(BP) isprovided as ballast capacitance so that a positive component (a noisecomponent with the same polarity as the input voltage V_(POW)) ofExpression 7 is the same as a negative component (a noise component witha reverse polarity to the input voltage V_(POW)).

In the right side of Expression 7, the first and third terms related tothe parasitic capacitances C_(SP) and C_(SO) and the first capacitanceelement C_(BP), are positive components, and the second term related tothe parasitic capacitance C_(SN) is a negative component. Here, in thefirst embodiment, when the first capacitance element C_(BP) is notprovided, the reciprocal (l/PSRR) of the PSRR is assumed to be smallerthan 0. In this case, the positive components related to the parasiticcapacitances C_(SP) and C_(SO) are less than the negative componentrelated to the parasitic capacitance C_(SN). Accordingly, by providingthe first capacitance element C_(BP) in parallel to the parasiticcapacitance C_(SP), it is possible to actually increase the positivecomponent. Thus, the linear regulator 1 according to the embodimentcauses the absolute value of the reciprocal (1/PSRR) of the PSRR to becloser to 0 than when the first capacitance element C_(BP) is notprovided.

In this way, in the linear regulator 1 according to the embodiment, thefirst capacitance element C_(BP) is connected between the power inputIN_(POW) and the first input IN1, and thus the noise mixed in the outputvoltage V_(O) can approach zero (almost cancelled) by the parasiticcapacitances C_(SN), C_(SP), and C_(SO). Thus, the PSRR characteristicsof the linear regulator 1 are improved.

FIG. 5 is a graph illustrating PSRR characteristics of the linearregulator 1 according to the first embodiment. The vertical axis of thegraph represents PSRR (dB). The horizontal axis represents a frequency(Hz) of power noise mixed in the output voltage V_(O).

A line L0 indicates the PSRR characteristic of a linear regulator inwhich the first capacitance element C_(BP) is not provided. A line L1indicates PSRR characteristics of a linear regulator in which the firstcapacitance element C_(BP) according to the first embodiment isprovided.

For example, referring to power noise with 10³ Hz frequency, the PSRR ofthe line L1 is higher by about 12 dB than the PSRR of the line L0.Accordingly, it can be understood that the PSRR characteristics can beconsiderably improved by providing the first capacitance element C_(BP)as in the first embodiment.

The capacitance of the first capacitance element C_(BP) at which theabsolute value of 1/PSRR approaches 0 can be determined by an actualmeasured value of the PSRR characteristics, a statistical averagedvalue, or a simulation. Since the capacitance of the first capacitanceelement C_(BP) is set to balance positive and negative components of thepower noise, a very small capacitance may be set. For example, thecapacitance of the first capacitance element C_(BP) set by a simulationdepicted in FIG. 5 is about 20 fF (femtofarads), a layout area of thelinear regulator 1 is not greatly increased, and an area penalty issmall relative to the improved performance. Accordingly, the linearregulator 1 according to the first embodiment has good PSRRcharacteristics and can be manufactured at low cost withoutsignificantly increasing a chip size. In the linear regulator 1according to the first embodiment, an increase in current consumption isavoided since the output impedances Z_(AMP) and Z_(DC) are not changed.

Second Embodiment

FIG. 6 is a circuit diagram illustrating an example of the configurationof a linear regulator according to a second embodiment. A linearregulator 1 according to the second embodiment includes a secondcapacitance element C_(BN). The second capacitance element C_(BN) isconnected between a power input IN_(POW) and a second input IN2. Thesecond capacitance element C_(BN) is provided as ballast capacitance toprevent noise being generated in an output voltage V_(O) of an amplifiercircuit AMP by parasitic capacitances C_(SN), C_(SP), and C_(SO) and toimprove a power supply ripple rejection ratio (PSRR) of the amplifiercircuit AMP. As with the first capacitance element C_(BP), the secondcapacitance element C_(BN) may be, for example, a MIS capacitor or maybe wiring capacitance as illustrated in FIGS. 2 or 3A and 3B. The otherremaining configuration details of the second embodiment may beconsidered the same as the corresponding configuration of the firstembodiment.

FIGS. 7A to 7E are graphs illustrating the waveforms of noise. FIG. 7Aillustrates noise of the input voltage V_(POW). FIG. 7B illustratesnoise transmitted via the parasitic capacitance C_(SP) in the outputOUT. FIG. 7C illustrates noise transmitted via the parasitic capacitanceC_(SN) in the second input IN2 and the output OUT. In FIG. 7C, a dottedline indicates the noise in the second input IN2 and a solid lineindicates the noise in the output OUT. FIG. 7D illustrates noisetransmitted via the parasitic capacitance C_(SO) in the output OUT. FIG.7E illustrates noise (ballast noise) transmitted via the secondcapacitance element C_(BN) in the output OUT. In FIG. 7E, a dotted lineindicates the noise in the second input IN2 and a solid line indicatesthe noise in the output unit OUT. The waveforms of the noise in FIGS. 7Ato 7E are depicted to facilitate the understanding conveniently and aretypically different from waveforms of actual noise.

As described with reference to FIGS. 4A to 4E, the phase of the noisefrom the parasitic capacitance C_(SN) is inverted at 180 degrees withrespect to the phase of the noise from the parasitic capacitances C_(SP)and C_(SO). Since the noise mixed in the output voltage V_(O) by theparasitic capacitances C_(SP) C_(SN), and C_(SO) is a sum of a curve inFIG. 7B, a curve in FIG. 7D, and a solid curved line in FIG. 7C, thenoise by the parasitic capacitance C_(SN) and the noise by the parasiticcapacitances C_(SP) and C_(SO) operate to cancel each other.

As described above, it is generally an accident and considerably rarethat the absolute value of the noise from the parasitic capacitanceC_(SN) is the same as the absolute value of the noise from the parasiticcapacitances C_(SP) and C_(SO) such that noise in the output voltageV_(O) would not be generated.

Accordingly, in the second embodiment, the second capacitance elementC_(BN) is intentionally provided as ballast capacitance to prevent noisebeing mixed in the output voltage V_(O) from the parasitic capacitancesC_(SP) C_(SN), and C_(SO). For example, when the absolute value of thenoise mixed in the output voltage V_(O) by the parasitic capacitanceC_(SN) is less than the absolute value of the noise mixed in the outputvoltage V_(O) by the parasitic capacitances C_(SP) and C_(SO), asillustrated in FIG. 6, the second capacitance element C_(BN) isconnected between the power input IN_(POW) and the second input IN2 andis provided in parallel to the parasitic capacitance C_(SN). Thus, aballast noise component indicated by the solid line in FIG. 7E isfurther added to the output voltage V_(O), and thus the total sum of thenoise mixed in the output voltage V_(O) is reduced in absolute value.

For example, in the second embodiment, NV_(P) is expressed by Expression8:

NV _(P) ={Z _(DC)/(Z _(DC)+1/(jωC _(SP)))}×V _(POW)  Expression 8:

Expression 9 is established based on Expressions 1 and 8:

NV _(OP)=(1+R ₂ /R ₁)×[{Z _(DC)/(Z _(DC)+1/(jωC _(SP)))}×V]_(POW)  Expression 9:

Here, a phase of NV_(OP) is advanced by 90 degrees from V_(POW) by theparasitic capacitance C_(SP).

When NV_(ON) is a noise component mixed in the output voltage V_(O) fromthe second input IN2 by the parasitic capacitance C_(SN) and secondcapacitance element C_(BN), NV_(ON) is expressed by Expression 10:

NV _(ON) =−R ₂/(1/(jωC _(SN) +C _(BN))))×V _(POW)  Expression 10:

Here, a phase of NV_(ON) is delayed by 90 degrees than V_(POW) (aninverted state with respect to the phase of NV_(OP)), as described. Thatis, when V_(POW) is a positive voltage, NV_(ON) is a negative voltagecomponent.

NV_(OO) is a noise component mixed in the output voltage V_(O) from theamplifier circuit AMP by the parasitic capacitance C_(SO), NV_(OO) isthe same as Expression 5 described above.

Here, a phase of NV_(OO) is advanced by 90 degrees from V_(POW) That is,when V_(POW) is a positive voltage, NV_(OO) is a positive voltagecomponent.

A noise component NV_(O) (NV_(OP)+NV_(ON)+NV_(OO)) generated in theoutput voltage V_(O) by the parasitic capacitances C_(SP), C_(SN), andC_(SO) and the second capacitance element C_(BN) is expressed byExpression 11:

NV _(O)={(1+R ₂ /R ₁)Z _(DC)/(Z _(DC)+1/(jωC _(SP)))−R ₂/(1/(jω(C _(SN)+C _(BN))))+Z _(AMP)/(Z _(AMP)+(1/(jωC _(SO)))}×V _(POW)  Expression 11:

In order to bring the noise component NV_(O) close to zero, the absolutevalue of the right side of Expression 11 may approach zero. In thesecond embodiment, a reciprocal (1/PSRR) of PSRR is expressed byExpression 12:

1/PSRR={(1+R ₂ /R ₁)Z _(DC)/(Z _(DC)+1/(jωC _(SP)))−R ₂/(1/(jω(C _(SN)+C _(BN))))+Z _(AMP)/(Z _(AMP)+(1/jωC _(SO)))}  Expression 12:

By bringing the absolute value of the right side of Expression close tozero, it is possible to obtain high PSRR characteristics.

In the second embodiment, the second capacitance element C_(BN) isprovided as ballast capacitance so that a positive component (a noisecomponent with the same polarity as the input voltage V_(POW)) ofExpression 12 is the same as a negative component (a noise componentwith a reverse polarity to the input voltage V_(POW)).

In the right side of Expression 12, the first and third terms related tothe parasitic capacitances C_(SP) and C_(SO) are positive components,and the second term related to the parasitic capacitance C_(SN) and thesecond capacitance element C_(BN) is a negative component. Here, in thesecond embodiment, when the second capacitance element C_(BN) is notprovided, the reciprocal (1/PSRR) of the PSRR is assumed to be greaterthan 0. In this case, the negative component related to the parasiticcapacitance C_(SN) is less than the positive components related to theparasitic capacitances C_(SP) and C_(SO). Accordingly, by providing thesecond capacitance element C_(BN) in parallel to the parasiticcapacitance C_(SN), it is possible to actually increase the negativecomponent. Thus, in the linear regulator 1 according to the secondembodiment, the absolute value of the reciprocal (1/PSRR) of the PSRRcan be brought closer to 0 than when the second capacitance elementC_(BN) is not provided. Thus, in the second embodiment, it is possibleto obtain the same advantages as those of the first embodiment.

The first and second embodiments may be combined.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the disclosure. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of thedisclosure. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the disclosure.

What is claimed is:
 1. A power circuit, comprising: an amplifier circuithaving a first input and a second input, the amplifier circuit receivingpower from a power input and outputting an output voltage correspondingto a voltage difference between the first and second inputs; a referencevoltage circuit that supplies a reference voltage to the first input ofthe amplifier circuit; a feedback circuit that supplies a feedbackvoltage corresponding to the output voltage to the second input of theamplifier circuit; and a first ballast capacitance element between thepower input and the first input of the amplifier circuit.
 2. The powercircuit according to claim 1, wherein the first ballast capacitanceelement has a capacitance value such that a reciprocal (1/PSRR) of apower supply rejection ratio (PSRR) of the amplifier circuit is closerto 0 than when the first ballast capacitance element is not provided. 3.The power circuit according to claim 2, wherein the feedback circuitcomprises: a first resistance element and a second resistance elementconnected in series between a low supply voltage terminal and the outputvoltage of the amplifier circuit, and a node between the first andsecond resistance elements is connected to the second input of theamplifier circuit.
 4. The power circuit according to claim 1, whereinthe first ballast capacitance element comprises: a first electrode on afirst planar device level and electrically connected to the first inputof the amplifier circuit; a second electrode on a second planar devicelevel and electrically connected to the power input; and a firstinsulation film between the first and second planar device levels. 5.The power circuit according to claim 1, wherein the first ballastcapacitance element comprises: a portion of a power wiring connectingthe power input to the amplifier circuit; a portion of a first wiringconnecting the reference voltage circuit and the first input of theamplifier circuit; and an insulation film between the portion of thepower wiring and the portion of the first wiring.
 6. The power circuitaccording to claim 5, wherein the portion of the power wiring and theportion of the first wiring are on a same planar device level.
 7. Thepower circuit according to claim 5, wherein the portion of the powerwiring is on a first planar device level and the portion of the firstwiring is on a second planar device level different from the firstplanar device level.
 8. The power circuit according to claim 5, whereinthe portion of the power wiring and the portion of the first wiring arein a serpentine pattern.
 9. The power circuit according to claim 5,wherein the portion of the power wiring and the portion of the firstwiring are straight line segments.
 10. The power circuit according toclaim 1, wherein the first ballast capacitance element comprises awiring capacitance formed between a portion of a first wiring thatconnects the power input to the amplifier circuit and a portion of asecond wiring connecting the reference voltage circuit to the firstinput of the amplifier circuit.
 11. The power circuit according to claim1, wherein the first ballast capacitance element comprises: a firstelectrode connected to a first wiring that connects the power input tothe amplifier circuit; and a second electrode connected to a secondwiring connecting the reference voltage circuit to the first input ofthe amplifier circuit.
 12. The power circuit according to claim 1,further comprising: a second ballast capacitance element between thepower input and the second input of the amplifier circuit.
 13. A powercircuit, comprising: an amplifier circuit having a first input and asecond input, the amplifier circuit receiving power from a power inputand outputting an output voltage corresponding to a voltage differencebetween the first and second inputs; a reference voltage circuit thatsupplies a reference voltage to the first input of the amplifiercircuit; a feedback circuit that supplies a feedback voltagecorresponding to the output voltage to the second input of the amplifiercircuit; and a first ballast capacitance element between the power inputand the second input of the amplifier circuit.
 14. The power circuitaccording to claim 13, wherein the first ballast capacitance element hasa capacitance value such that a reciprocal (1/PSRR) of a power supplyrejection ratio (PSRR) of the amplifier circuit is closer to 0 than whenthe first ballast capacitance element is not provided.
 15. The powercircuit according to claim 13, wherein the feedback circuit comprises: afirst resistance element and a second resistance element connected inseries between a low supply voltage terminal and the output voltage ofthe amplifier circuit; and a node between the first and secondresistance elements is connected to the second input of the amplifiercircuit.
 16. The power circuit according to claim 13, wherein the firstballast capacitance element comprises: a first electrode on a firstplanar device level and electrically connected to the second input ofthe amplifier circuit; a second electrode on a second planar devicelevel and electrically connected to the power input; and a firstinsulation film between the first and second planar device levels. 17.The power circuit according to claim 13, wherein the first ballastcapacitance element comprises: a portion of a power wiring connectingthe power input to the amplifier circuit; a portion of a first wiringconnecting the node of the feedback circuit to the second input of theamplifier circuit; and an insulation film between the portion of thepower wiring and the portion of the first wiring.
 18. The power circuitaccording to claim 17, wherein the portion of the power wiring and theportion of the first wiring are on a same planar device level.
 19. Thepower circuit according to claim 13, wherein the first ballastcapacitance element comprises a wiring capacitance formed between aportion of a first wiring that connects the power input to the amplifiercircuit and a portion of a second wiring connecting the node of thefeedback circuit to the second input of the amplifier circuit.
 20. Thepower circuit according to claim 13, further comprising: a secondballast capacitance element between the power input and the first inputof the amplifier circuit.